The simplest definition of a multi-chip module (MCM) is that the integrated circuit module (or chip carrier) has more than one chip on it as shown in FIG. 1. An explosive growth in the research and development efforts devoted to MCM for the purpose of breaking through the density and performance limitations of single chip modules has been witnessed in the past few yeas. A multi-chip module may combine many high-performance integrated circuit chips with a custom designed substrate structure to take full advantage of the integrated circuit performance. As an example, Fujitsu's MLG-MCM module contains 144 integrated circuit chips. The complex substrate structure is the heart of the MCM technology. It can be fabricated by multi-layers of ceramic material, polymer, silicon, metal, glass ceramic material, printed circuit boards (PCB), etc., using thin films, thick films, co-fired, or layered methods.
A formal definition of MCM has been given by the Institute for Interconnecting and Packaging Electronic Circuits (IPC). They define three main categories of MCMs as follows:
1. MCM-C: A module uses thick film technology such as fireable metals to form conductive patterns, and is constructed entirely from ceramic or glass-ceramic materials, or possibly, other materials having a dielectric constant above 5. In short, a MCM-C is constructed on ceramic (C) or glass-ceramic substrate. PA0 2. MCM-L: A multi-chip uses laminate structures and employs printed circuit board (PCB) technology to form predominantly copper conductors and vias. These structures may sometimes contain thermal expansion controlling metal layers. In short, MCM-L utilizes PCB technology of reinforced plastic laminates (L). PA0 3. MCM-D: A module has a substrate on which the multi-layered signal conductors are formed by the deposition of thin-film metals on unreinforced dielectric materials with a dielectric constant below 5 over a support structure such as silicon, ceramic material, or metal. In short, MCM-D uses deposited metal and unreinforced dielectric on a variety of rigid substrate base.
The techniques of chip on board (COB) have been widely used for packaging multi-chip modules. COB is defined as the direct attachment of bare chips on a PCB or substrate by wire bonding, tape automated bonding (TAB), or flip chip attachment (FCA). As shown in FIG. 2, a substrate or printed circuit board 201 is used for attaching integrated circuit chips on it. A first circuit chip 202 is attached to the substrate or printed circuit board 201 by wire loops 203 using wire bonding technology. A second circuit chip 204 is attached to the substrate or printed circuit board 201 by tap leads 205 with tape automated bonding technology. A third chip 206 is flipped and connected to the substrate or printed circuit board 201 by solder balls 207 using flip chip attachment technology. FCA, which uses smaller solder balls as the joint structure, has the following advantages over other types of packaging technology:
Higher density I/O connections by area array joints. PA1 Superior electrical performance due to small bumps for joints. PA1 Multi-chip attachment by a single reflow of solder bumps.
For most interconnection traces on a MCM substrate, the loads are mostly capacitive and on the order of a few pFs. The trace load can be so small as to be comparable to some of the heavily loaded lines found on-chip. These interconnection traces can thus be treated as an extension of the on-chip network. Due to the small trace-load, the sizes of the MCM I/O buffers, which are designed in chip and connected to these traces, can be scaled down in chip-to-chip communication.
Currently, in MCM designs, silicon chips with conventional I/C buffers (CIOB) have been used in a FCA configuration. The main reason is that these chips have already been fabricated and are available for immediate use. Nevertheless, CIOBs for these chips are not well suited for a MCM environment. The performance of the MCM is not optimized and certain features are degraded. These features include increased power dissipation and increased chip-to-chip delay. In addition, conventional I/O buffers are usually designed with electrostatic discharge (ESD) protection structure. For some MCM I/O buffers, the ESD structure is unnecessary because the buffers are only used for the chip-to-chip communication within a MCM package.
As technologies of both MCM and FCA continue to improve, they offer features such as low inductance, reduced size and low parasitic capacitance. All these features provide a prospective future for VLSI system with higher speed and smaller size on a MCM module. However, for a better system that is optimized with a MCM packaging environment, some improvements can be achieved in the chip design.
A differential type of I/O buffers (DIOB) used as miniaturized I/O buffers in a MCM-FCA environment was presented by T. Gabara et al. in "An I/O Buffer Set Silicon Multi-Chip Module (MCM)", pp. 147-152, IEEE Multi-Chip Module Conference, MCMC-93, March, 1993. Although DIOB provides a good operation speed and noise margin, the additional bump pads required may reduce the yield rate of manufacturing the MCM. As the high-end packaging techniques such as MCM-D remain a market trend, the need for specialized I/O buffers is necessary for achieving high performance at low power dissipation.